The workaround described in SOL15034 requires an engineering hotfix which introduces that db variable:
To work around this issue, you must obtain an engineering hotfix for this issue and install it on the affected BIG-IP system. The engineering hotfix introduces a new quickassist.compression.buffsize_multiplier database variable that you must configure its value to 300. To obtain an engineering hotfix for this issue, contact F5 Support. To modify the quickassist.compression.buffsize_multiplier database variable, perform the following procedure...