When miners receive new block they start working on empty block so they are guaranteed to have valid block. When they finally validate the received block they can add transactions and mine for nonempty blocks. It might take about 30 seconds to validate which fits 5% of empty blocks.

I see where you aiming, if bigger blocks are received then it takes longer to validate thus higher % of empty blocks. But there is work in progress where miners could anounce the block with txs they are mining on, or adding their own txs which noone else can known about (like payments to its users), or naturally increased CPU processing speed over time, or another possibility is dedicated ASIC hardware for faster validation if fees become really important and other options fail.

But it is only worries for miners who are expected to have better hardware than regular nodes who are ok even with medium system hardware specs when the block size limit is increased.