a few tips to better the performance
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turn off (Disable) modifiers while painting, (If you need them back just turn them on and try to set their values as low as possible while painting, but again turning them off or disabling gives you the best, )
If its still laggy, try paiting click by click, hold the mouse for a sec while clicked once, if you move the mouse the brush will keep painting even while lagging, and thats why sometimes it lags even more than you expected