TTS API comparison 2026
Gradium TTS vs Fish Audio S2.1 Pro Gradium vs Fish Audio · text-to-speech for voice agents
Gradium TTS is in the Headroom speed tier (P95 at or under 101 ms); Fish Audio S2.1 Pro is in the Visible delay speed tier (P95 of 320 ms or more). Below, each measure side by side with its source; a model leads on a measure only beyond the tie rule. No combined score, no overall winner.
Data read October 22, 2026 · Coval, Speko, Artificial Analysis
Side by side
| Measure | Gradium TTS | Fish Audio S2.1 Pro |
|---|---|---|
| Speed tierCoval P95, 30 days | Headroom | Visible delay |
| Median time to first audioCoval, 30-day window | 55 ms | 318 ms |
| P95 time to first audio (slow turns)Coval, 30-day window | 101 ms | 568 ms |
| Time to first audio, board chartSpeko, US East | 55 ms | 198 ms |
| Naturalness EloSpeko | 1,592 | not rated |
| Listener Elo (95% interval)Artificial Analysis | 1,176 ±13tie | 1,189 ±14tie |
| Numbers and dates (0 to 1)Speko | 0.27 | not rated |
| List price per 1M charactersCoval, Speko, Artificial Analysis | $35.9 to $57.8 | $15 |
In bold: the model ahead on that measure. “tie”: latencies within 10 ms, or Elo scores whose 95% intervals overlap.
Where Gradium TTS is ahead
- →Median time to first audio: 55 ms vs 318 ms (Coval, 30-day window)
- →P95 time to first audio (slow turns): 101 ms vs 568 ms (Coval, 30-day window)
- →Time to first audio, board chart: 55 ms vs 198 ms (Speko, US East)
Where Fish Audio S2.1 Pro is ahead
- →List price per 1M characters: $15 vs $35.9 to $57.8 (Coval, Speko, Artificial Analysis)
Frequently asked questions
- What is the difference between Gradium TTS and Fish Audio S2.1 Pro?
- Gradium TTS (Gradium) is in the Headroom speed tier (P95 at or under 101 ms); Fish Audio S2.1 Pro (Fish Audio) is in the Visible delay speed tier (P95 of 320 ms or more). Gradium TTS leads on median time to first audio, P95 time to first audio (slow turns) and time to first audio, board chart; Fish Audio S2.1 Pro leads on list price per 1M characters. They are a tie on listener Elo (95% interval). No combined score is computed.
- Which is faster for real-time voice agents, Gradium TTS or Fish Audio S2.1 Pro?
- On Coval's 30-day window read on October 22, 2026, the median time to first audio is 55 ms for Gradium TTS and 318 ms for Fish Audio S2.1 Pro (Gradium TTS ahead by 263 ms); the P95, the slow turn, is 101 ms and 568 ms (Gradium TTS ahead by 467 ms). Gaps under 10 ms are ties.
- Which sounds more natural, Gradium TTS or Fish Audio S2.1 Pro?
- Speko naturalness: 1,592 for Gradium TTS, not rated for Fish Audio S2.1 Pro. Artificial Analysis listener Elo: 1,176 ±13 and 1,189 ±14, a tie because the 95% intervals overlap. The two sources use different listeners and can disagree.
- How are these two text-to-speech APIs compared?
- Speed first, on Coval's median and P95 time to first audio; then voice quality (Speko, Artificial Analysis), accuracy on numbers and dates (Speko) and list price. Latencies within 10 ms and Elo scores whose intervals overlap are ties. Scores from different benchmarks are never added into one number.
- Can these numbers be checked?
- Yes. Each model page lists every measurement with its source and the date of the reading, and links to the benchmark page it comes from. The rules are on the methodology page.