schmitt trigger delay
You may have better luck using an CMOS or HCMOS version of the '14. The input impedance of a 74LS14 is fairly low (< 10K). A 74HC14 (or 74C14/CD4584) has a much higher input impedance, and the nominal switching threshold is also closer to one RC time constant (3.25V/5V = 0.65; 1 TC = 0.632).
Note that the 74HCT14 has TTL level input thresholds, so the low-high switching point would be closer to 1/2 time constant (1.8V/5V = 0.36; 0.5 TC = 0.393). I'd use the HC version for the extra range.