There are four possible causes that I can think of:
1. An inadequate download speed. (You seem to have eliminated that possibility) ;
2. A poorly-specified computer. (That seems to be irrelevant here too) ;
3. Something on your computer is hogging most of its resources. (That's certainly a possibility. Malware can do it but other software, particularly anti-virus programs, can be the cause). We might need to investigate that further if the following doesn't help . . .
4. You need to clear your browser's cache. (It's a common problem with repeated use of streaming services, whether that be Netflix or something like Youtube). Instructions here, if you need them:
https://www.digitaltrends.com/computing/how-to-clear-your-browser-cache/