There is an unfortunate consequence of the design of the Slimcoin burn approach - coins are burned in a transaction and any unspent burn rewards will qualify as staking coins and if allowed to stake, those stake rewards themselves will qualify as staking coins, which if allowed to stake ... and so on, and so on. Setting a reservebalance merely limits the amount of stake processing, it doesn't stop it entirely. The computational demand of stake processing (effectively, continuously calculating the UTXO set) isn't such a significant factor with Peercoin with its shorter blockchain but with Slimcoin the (not entirely unpredictable) computational consequences of a 90s block time on the PoS staking calculations are starting to kick in quite sharply for those with well-funded wallets. It's not even clear whether an update to a later clone of Bitcoin would provide any actual solution - Bitcoin, being pure PoW, has no specific UTXO calculation optimisations that the PoS staking calculations could benefit from.