The security of the blockchain relies in the fact that new blocks are added faster than any attacker guy can add them (as long as <51%)
In case the frequency of the bitcoin transactions becomes less, an attacker could use the time to remove some previous transactions (not too far behind) and do bis proof of work. Then... When new Block is finally verified and ready to be added, in the case he is the fastest miner (even <51%), he could propagate his bogus blockchain fork.
Of course, due to <51% his chances are little but because of the low amount of transactions, he could try over and over again (thus increasing his chances).
World this render blockchain more insecure?