Is there any accelerating effect on the EPO’s grant proceedings by choosing the EPO as ISA?

Yes. Where the EPO acted as ISA, the supplementary European search report is dispensed with (Article 153(7) EPC): Since the EPO performs international searches as ISA to the same high-quality standard as a European search, applications already searched by the EPO during the international phase do not have to undergo a supplementary European search upon entry into the European phase. In practice, this means that, by choosing the EPO as ISA, you can save up to 12 months in time during the European phase and benefit financially because you do not need to pay the supplementary European search fee, normally payable when entering the European phase.

For more details about expediting the grant proceedings, see “Options available for expediting the proceedings before the EPO as designated or elected Office.”