Do you by any chance have a lot of filters, specially cosmetic filters? Any scripts like say YouTube+ or anything that is used for media extraction (save video) because if a lot of things are injected into the page, it can cause memory leaks and performance issues.
Also, some sites, I have experienced it myself, will cause things to slow down because uB0 is doing its job. For example, they are the kind that ticker style constantly use Ajax to load/lazy load ads into the page or stream, and since uB0 blocks on the network level, each time this happens, you will notice a little degradation and hit and it can add up on the same tab; specially worse if you have multiple tabs open in which this is happening. Just some things to consider.