I mean it has 333mhz ao I thought it would play without to much of a problem
Going from 128MB to the full 256MB allowed by that system probably won't make much of a difference in playing Youtube videos with a 333Mhz Pentium II (a 10-12yo CPU).
Was the video you were trying to watch HD video? If so switching out of HD should help.
If you don't mind my asking, how much did you end up paying for that Thinkpad 390e