SFZ Instruments lag quite a lot (MuseScore 3.6.2)

I have installed Ivy Audio's Piano in 162 SFZ SoundFont. When I use it, playback can sometimes lag a lot and other times not so much, even when playing back the same score.
It should be because I'm using an old computer (late 2009 intel mac, 4gb RAM).
Is there any way to reduce this lag?

Thanks in advance.