My only experience with Efke 100 has been with R100 (120 version) exposed at 100 and developed in Neofin Blau. Resolution was very crisp and grain well within acceptable bounds - tighter and finer than Rodinal.
The speed appears to be right at 100. I got good shadow detail and excellent midtones. It appears to have many of the qualities I enjoy in 120 APX 100.
The presoak effluent poured off a heavy dark greenish blue color. Only one shot was aimed into strongly backlit trees indicating the anti-halation treatment is probably necessary - there was some halation spillover into the margins.
You might try a roll in a developer like Rodinal, a Beutler type or something similar.