2160p anime lags due to subs

[maxplosive]

So when I play anime with ASS subs I see a message that the subs are burning in and to wait a few seconds and it works fine with all shows except for those in 2160p because then they lag every now and then, is this because my CPU is too weak? I'm currently just running the server on an old laptop but planning on building a NAS later this month and was looking at a Topton mobo with a celeron N5105, would that CPu fix this issue?