EPUB files take ages to open

Hi all, I tried searching the web for a solution to no aid. Issue is that EPUB files (an ebook file format) take a very long time to open, and even after opened, browsing pages inside the epub is slow. It does not matter if I open the book with Calibre or SumatraPDF, I even tried different extensions inside Firefox or Edge, it's always slow, sometimes taking up to 2 or 3 minutes to open, and SumatraPDF sometimes not even opening and staying in task manager as a high CPU process forcing me to terminate it. Disabling Windows Defender is of no help and changing the Graphics Performance preference (GPU assisted or disabled) does not fix it. Anyone with a hint will be greatly appreciated!

Could the epub files be damaged?
One thing you could try is one of the online conversion sites, convert to mobi, then convert the mobi back to epub
Convert text and ebooks to the MOBI format

I've had some free epubs myself. I usually upload them to google so I can read in Play Books on my phone.
If they open in there, they're good

antares said:

It does not matter if I open the book with Calibre or SumatraPDF, I even tried different extensions inside Firefox or Edge, it's always slow, sometimes taking up to 2 or 3 minutes to open, and SumatraPDF sometimes not even opening and staying in task manager as a high CPU process forcing me to terminate it.

Calibre:
Q. Latest version?
Q. 64-bit or 32-bit?
Q. Installed or portable?

SumatraPDF:
Q. Latest version?
Q. 64-bit or 32-bit?
Q. Installed or portable?

Have you used Sysinternals/Technet's Process Monitor before (for example, to search for FAILURE events)?

Are these Epub large, medium or small filesizes or doesn't it matter?

Do you have sufficient free hard disk space, for example - for the temporary folder Calibre creates for each opened epub file?

davexnet said:

Could the epub files be damaged?
One thing you could try is one of the online conversion sites, convert to mobi, then convert the mobi back to epub
Convert text and ebooks to the MOBI format

I've had some free epubs myself. I usually upload them to google so I can read in Play Books on my phone.
If they open in there, they're good

Yes, I can try Epub Converter too and convert them to other formats like PDF, etc, but conversion is not perfect, you lose the original index, etc

RickC said:

Calibre:
Q. Latest version?
Q. 64-bit or 32-bit?
Q. Installed or portable?

SumatraPDF:
Q. Latest version?
Q. 64-bit or 32-bit?
Q. Installed or portable?

Have you used Sysinternals/Technet's Process Monitor before (for example, to search for FAILURE events)?

Are these Epub large, medium or small filesizes or doesn't it matter?

Do you have sufficient free hard disk space, for example - for the temporary folder Calibre creates for each opened epub file?

Calibre is latest version, 64bit, same with Sumatra. As I said it happens with any Epub regardless of size, etc. And yes, I have more than enough free space. I will try Process Monitor to look for some hidden process when opening the Epubs, maybe it shows a deeper level than the regular Windows Task Manager, thanks

antares said:

I will try Process Monitor to look for some hidden process when opening the Epubs, maybe it shows a deeper level than the regular Windows Task Manager

Process Monitor (ProcMon) will show a much deeper level than Task Manager - registry, filesystem, network and process activity - but has a learning curve because of the huge amount of activity entries it captures... thousands every minute. IMO the secret to using ProcMon is filtering both events and results.

Have a look at this tutorial for more info: Understanding Process Monitor
(The link is to the Wayback Machine because the How-To Geek website has moved or removed the tutorial.)

I suggest examining ProcMon for the SumatraPDF process and/or filepath because the main calibre.exe process also invokes multiple calibre-parallel.exe and conhost.exe processes, thus making it more difficult to spot issues.

I also suggest creating a shortcut to run ProcMon with the following two properties:

(a) is obvious, i.e. to run ProcMon as Administrator and (b) is so ProcMon doesn't start capturing automatically.

How-To example based on using the portable version of Sumatra (which is just a single executable) with an epub file, both located on a USB drive (D:):


1. Run ProcMon from the new shortcut and accept the UAC prompt. (You will also have to accept a EULA the first time it is run.)

2. Run SumatraPDF then position the Windows for Procmon and Sumatra side-by-side to make it easier to swap between the two.

3. Swap back to ProcMon and click on the Capture icon in the toolbar (or press CTRL+E) to start capturing.

4. Swap back to SumatraPDF and try to open an epub file.

5. After a few seconds of the epub file failing to open, swap back to ProcMon and click on the Capture icon in the toolbar (or press CTRL+E) to stop capturing.

6. Click on ProcMon's Find icon and search for SumatraPDF. Once the process has been found, right-click on an entry in the Path column and choose Include. This will filter the results to only include the filepath. You will see in the Process Name column all the processes interacting with SumatraPDF. The most important column is Result. You can ignore entries marked as SUCCESS. Instead, look for results that show errors like FAILURE, ACCESS DENIED, FILE NOT FOUND, etc.

Note: You can also use the Find icon to look for the epub's filename. This may show errors quicker than looking for SumatraPDF.

Hope this helps...