Thanks for all the replies.
The video I watch is mainly 720P or 1080P streaming from various sources. iPlayer and others are mostly OK, so it isn't a huge problem, just something that catches my eye from time to time.
The PC is about a year old, and is an AMD Athlon 3000G with Radeon Vega Graphics. I had to resort to the internal graphics when upgrading to Windows 11 as my old graphics card didn't meet the required criteria.
Still keen to hear from any users of Windows 11 what graphics cards they use, please.
Alternatively, can the internal graphics be tweaked to use more memory? I haven't found a setting/option that allows this, so would be interested in any advice on that score.