External SSDs not suitable for large file transfers?

I recently returned a PNY Elite portable SSD ( PNY Portable SSD Elite Silver USB 3.1 (480GB): Amazon.co.uk: Computers & Accessories ) because the transfer rate fell a lot after about 60 seconds. I was writing 32GB and 62GB image files from internal SSD to it and the transfer rate (via USB 3.0 SATA) maintained about 350MB/s for the first 60 seconds but then fell dramatically to about 24MB/s. Writing to my other (Samsung M3) external HDD maintained 100MB/s throughout and I was expecting the PNY SSD to maintain 350MB/s throughout. See pictures. I suppose if I was writing only 5GB and it took less than 60 seconds then I would not have the problem but the reason I want the SSD is to write images up to 80GB to it.

Question - Do all external SSDs suffer this dramatic fall in transfer rates after 60 seconds or have I been unlucky in buying the PNY? If so then SSDs are worse than external HDDs for large file storage.

Switched to USB2 transfer rate; the cause can be any aspect of USB3: hw incompatibility / hw failure / bad driver / insufficient power supply. Try other USB enclosure.

SSDs use less power than HDDs so I don't think it is a power supply issue but it may be power related in that the SSD did get quite warm. Maybe there is an in-built safety algorythm to slow down the transfer rate after 60 seconds to stop the SSD from getting too hot and damaging it? Just a guess on my part but if it is correct then no SSDs are suitable for writing large sequential data to. I am attaching another picture which suggests this might be true. .
Maybe others may know different.

It is the overall design of the particular SSD used, controller, memory cells and size of extra buffer memory. Samsung are of course the market leaders in that respect.

PNY are the cheap end of the market. It probably lacks in a decent size of buffer or some thermal limitation which leads to that drop in performance.

It is mentioned in at least 2 of those Amazon reviews.
Quite well known with certain SSD drives.

I recently bought a USB-C 4 TB drive of the standard HDD 5,400rpm type and that was a good 100 MB/s with the test I did with real files to backup.
Makes you think as it costs the same as an SSD but 8x the capacity.

Seems to me that the average SSD is only suitable for 'small' amounts of data writing. I know 'small' is a relative phrase but I reckon anything above 15GB and you are better off writing to an external HDD than an external SSD.
Of course one could buy a top-of-the-range SSD costing a lot but is this a garauntee that high speed writing will be maintained throughout with no speed fall-off? You could pay a lot and still have the same problem.

It is the overall design of the particular SSD used

Imo no existing SSD has sustained transfer rate as low as 25MB/s; 300MB/s is minimum. Afaik the problem is related to that enclosure/cable/port, not to SSD drive inside.

wiganken: Use Crystal Disk Info to see which SSD is inside the enclosure. Test it by connecting to different port/comp using different cable(s). Connect it to USB2 port & observe the speed.

"Use Crystal Disk Info to see which SSD is inside the enclosure" - I can't do this as I no longer have the PNY SSD. I returned it but I decided to carry out a test: -

My PC has two internal SSDs. Drive0 = Crucial MX500 250GB SSD and Drive1 = Crucial MX500 500GB SSD. Both these are connected to the internal SATA 3 bus which is good for 600MB/s. I copied a 60.5GB Macrium image file from Drive1 to Drive0 after ensuring there was 144GB of free space. A similar thing happened in that the transfer rate (TR) stayed at 431 MB/s for 1min 20secs then fell to 181 MB/s. At 2mins it went back up but only to 200 MB/s and for the last 15 seconds it fell to only 60 MB/s. There was no USB involved so it seems to me that there is some heat protection built into the SSD which reduces the speed if it gets too hot. Just my guess but it seems that HDDs are best for large data writes.

Could be. It´s called "Thermal throttling", Thermal Throttling - Technology - Apacer for Industrial - The most reliable storage and memory

SATA III limits an SSD to a maximum of around 550 MB/s. So your numbers look correct.
Crucial(Micron) are one of the better manufacturers so you would expect a decent performance.

Some SSDs may use the SSD memory for the Cache/Buffer up to 25% of capacity or something like that, so when getting full starts to slow up. That, plus thermal effects where the drive has to be slowed up to keep within whatever the working temperature range is, would account for the decrease in speed.

Rather CPU thermal throttling than SSD one... or other CPU/mobo problem. Copying 100+ GB file between SSDs without problems.